In recent years, more stringent regulations and greater appreciation of the importance of environmental preservation and safety have led to increased demands for adhesives containing no solvents, and even in the field of laminated composite films containing a metal foil, which are used in high-temperature sterilization treatments, the removal of solvents from the lamination adhesive used in bonding the metal foil to any of various plastic films is also being investigated.
Conventionally, two-liquid adhesives containing a polyol compound as the main agent and a polyisocyanate compound as the curing agent have been widely used as the solvent-based lamination adhesives used in these types of laminated composite films containing a metal foil, and the introduction of acid groups at the molecular terminals of the polyol compound and the inclusion of phosphoric acid as an additive are widely known techniques for imparting the metal foil with resistance to contents (acid resistance) (for example, see Patent Document 1 or 2).
On the other hand, in terms of solvent-free adhesives, in order to address the problem of increased viscosity due to the removal of solvents, and ensure satisfactory adhesive strength and good acid resistance to contents following curing, a method in which a two-liquid solvent-free adhesive containing a low-molecular weight polyol compound and a polyisocyanate compound is applied to the substrate under high-temperature conditions is generally used.
As a result, compared with solvent-based adhesives, solvent-free adhesives tend to have a much faster curing reaction rate, and for example if the solvent-free adhesive is used as a lamination adhesive for a high-temperature sterilization treatment, and acid groups are introduced into the main agent for the purpose of imparting the metal foil with acid resistance, then a problem arises in that curing is accelerated even further due to the urethanization catalytic effect of the acid groups, meaning the viscosity increase that occurs following mixing of the main agent and the curing agent is very rapid, resulting in a marked deterioration in workability.
In response to this type of problem, for example, Patent Document 3 proposes a method of ensuring that at least 30% of all the hydroxyl groups within the main agent polyol compound are secondary or tertiary hydroxyl groups, thereby lowering the hydroxyl group reactivity, and discloses that even when acid groups are introduced into the main agent, viscosity increases can be suppressed, and a solvent-free adhesive having good acid resistance can be obtained.
However, the reactivity of a secondary or tertiary hydroxyl group with an isocyanate group is generally similar to, or less than, that of water, and therefore the adhesive is prone to the effects of moisture in the air during curing, and for example if the adhesive application operation and aging operation are performed under high humidity conditions, then a problem arises in that because the amount of reaction between the isocyanate groups and water increases compared with the case of low-humidity conditions, the adhesive strength and the acid resistance tend to deteriorate.
Further, Patent Document 4 discloses that by using a diisocyanate-terminated prepolymer and an allophanate-modified diisocyanate obtained by reacting a portion of the isocyanate groups with an alcohol, the contents resistance following a heat sterilization treatment can be improved, and favorable pot life and workability can be achieved.
However, if an allophanate-modified diisocyanate is used, then a problem arises in that heat sterilization over a long period causes a deterioration in the adhesive strength.